This invention relates to improvements in joint prostheses. It is disclosed in the context of a shoulder prosthesis but is believed to have utility in other applications as well.
There are several prostheses that employ, as part of their systems for joining prosthesis components, tapered locking joints such as Morse tapered joints and the like. There are also the systems illustrated and described in U.S. Pat. Nos.: 5,080,685; 5,181,928; 5,314,479; 4,932,974; and 4,624,674; European Patent Specification 0,547,369 B1; Buechel-Pappas.TM. Total Shoulder System Implants and Instruments; and, Buechel-Pappas.TM. Total Shoulder System Surgical Procedure by Frederick F. Buechel M.D. No representation is intended that a thorough search of all material prior art has been conducted or that no more material prior art exists. Nor should any such representation be inferred. The disclosures of the '685, '928 and '479 patents are incorporated herein by reference.
A significant concern in the design of joint prostheses is weight. Particularly the bearing surfaces of such prostheses must be made of durable materials, with such alloys as cobalt chrome being preferred by many physicians. Titanium, while it is lighter, is not as durable as cobalt chrome. So, titanium is not an acceptable substitute for many prosthetic joint bearing surfaces. The generally competing desires for light weight and durability have led to several multiple component prostheses. In such prostheses, components that do not provide bearing surfaces are fabricated from somewhat less durable but typically more lightweight materials such as, for example, titanium. Components that provide bearing surfaces are fabricated from somewhat more durable but typically heavier weight materials such as, for example, cobalt chrome. A problem with many such prostheses is that they are designed without optimal regard for the durability-weight tradeoffs. Consequently, many bearing components are designed for greater strength than is necessary under the circumstances. An example would be a shoulder prosthesis, the head member of which is constructed from cobalt chrome and the body/shank element of which is made from titanium alloy. Although the head is constructed from extremely durable cobalt chrome, it is a thick-walled component, making it heavier than necessary to perform its bearing function. The head's thick-walled design is related in part to the manner in which it is joined to the body/shank element of the shoulder prosthesis. Usually this involves multiple butt joint and/or tapered surfaces and threaded fasteners, all of which add to the weight of the prosthesis.
Another significant consideration in the design of such joint prostheses is range of motion. Designs strive to achieve the natural range of motion of a healthy joint. How the head member and body/shank element are joined in the shoulder prosthesis example described above, clearly affects the range of motion available in said prosthesis. In a few of the shoulder joint prostheses identified above, for example, some potential bearing surface on the humeral component's head is lost. Such loss is due to the technique used to connect the head to the body/shank element of said humeral component.
Yet another significant consideration in the design of such joint prostheses is the positioning of the head on the body/shank portion to achieve a natural shoulder joint alignment. This alignment varies from patient to patient. Therefore, some designs have securing pins that fix an eccentric alignment of the head upon a spacer. See for example U.S. Pat. No. 5,358,526. These pins can add to the weight of the head. What is needed is a modular shoulder prosthesis that has a lighter weight head member whose alignment on the shank element can be adjusted easily by the caregiver to provide a custom-fit orientation for each patient and that itself cooperates with the body/shank to lock into a secure and fixed position.
It is an object of the invention to provide a lighter weight configuration for a prosthesis.
It is another object of the present invention to provide a design for a multiple component joint prosthesis that makes more effective use of potentially available joint bearing surface.
It is another object of the invention to provide a multiple component joint prosthesis that has a head member formed for eccentric alignment on a shank/body element.
Yet another object of the invention is to provide a multiple joint prosthesis that has a collar that prevents sliding movement of the prosthesis into the medullary passageway of a bone and taper-locks a head member in a fixed position thereon.
It is still another object of the present invention to provide a joint prothesis having a head member formed for eccentric alignment on a shank/body element and for taper-locking engagement on the shank/body element.
According to an aspect of the invention a kit for assembly of a modular joint prosthesis that can replace a head, neck, and adjacent portions of a bone of the joint comprises at least two shank/body elements. Each element is sized for insertion into the shank of the bone. At least two head members are also provided. Each head member is sized to replace one bearing surface of the joint. In addition, each shank/body element comprises an upper collar that lies superimposed on the resected level of the bone in the completed prosthetic joint. The collar provides a male taper at its outer diameter. Each head member has a first geometric axis therethrough. Moreover, the head member defines a part spherical cavity and includes a skirt extending about the cavity. The skirt provides a complementary female taper for receiving the male taper of each shank/body element.
A second geometric axis extends through the skirt along its central geometric axis. The first and second axis may be coincident in accordance with the present invention. The second axis is, however, offset from the first axis in preferred embodiments to enable the head member to undergo eccentric alignment on the collar. Engagement of the female taper on the male taper provides a mechanical connection between the head and the shank/body element that may be the sole mechanical connection in the preferred embodiments. It will be appreciated, however, that one skilled in the art may find additional techniques for supplementing the taper lock connection without departing from the scope of this invention.
According to another aspect of the invention, a joint prosthesis consists essentially of an integral shank and body. The body includes a collar having a male taper toward a complementary bearing surface to the prosthesis. A head has a geometric axis therethrough and a female taper defining a second geometric axis that may be coincident with the first axis or offset from the first axis in preferred embodiments. The female taper is complementary to the male taper of the collar to receive the collar in the assembled joint.
According to another aspect of the invention, a joint prosthesis comprises a first component and a second component. This second component consists essentially of an integral shank and body. The body includes a collar having a male taper toward the first component in the assembled joint. The second component consists essentially of a head having a geometric axis therethrough and a female taper complementary to the male taper of the collar. The female taper may be in concentric alignment with the head member about the geometric axis. However, the female taper is offset from the geometric axis in preferred embodiments. This female taper is formed to receive the collar in the assembled joint.
According to another aspect of the invention, a joint prosthesis assembly that can replace a head, neck, and adjacent portion of a first bone of the joint removed at a resected level comprises a head member and a shank/body element. That element comprises a body element with a shank element extending downwardly therefrom. The shank element is formed to be received in a shank of the resected first bone. The body element includes a collar portion to be adjacent the resected level. The collar portion has a radially outer peripheral surface. This outer surface provides a peripheral engaging surface extending away from the resected level toward an opposite bearing surface of a second bone of the joint. A head member provides a spherical outer bearing surface facing the opposite bearing surface and having a geometric axis extending therethrough. The head member also includes an inner surface having, at its radially outer extent, a corresponding peripheral surface. This peripheral surface engages the collar engaging surface after the shank/body element is installed in the shank of the bone. The peripheral engaging surface also defines a second geometric axis therethrough. The second axis may be coincident with the first axis or, in preferred embodiments, offset and substantially parallel to the first axis. The engaging surfaces provide a mechanical connection between the head member and the shank/body element that may be the sole connectors.